In the supply of lubricating oil for a transmission gear mechanism, lubricating oil supplied from an oil pump passes through a suitable fixed oil passageway, is guided to an oil passage formed inside a transmission case wall section and a thick part of a transmission case cover (transmission holder), passes through an oil supply passage passing through inner parts of a main shaft and a counter shaft (drive shaft), and is then sequentially supplied to bearing sections of the shafts and reduction gear train mounting sections.
As shown in FIG. 8 and FIG. 9, a transmission gear mechanism disclosed in Japanese patent No. 2655407 has a structure where a primary drive gear 01a provided on one end of an engine crankshaft 01 meshes with a primary driven gear 02c on a counter shaft (main shaft) 02, with rotation of the crankshaft 01 being transmitted to the counter shaft 02 via a switching clutch 02d interposed between the primary driven gear 02c and the counter shaft 02, and also transmitted to a drive shaft (counter shaft) 03 by means of meshing of reduction gear trains 02e and 03e. Reference numeral 03s is a drive sprocket.
The lubrication system for a transmission gear mechanism is configured such that lubricating oil force-fed from a lubricating oil pump 010 passes through a fixed oil passage 04 (refer to FIG. 9), through an oil passage such as inside a wall section of a transmission case OM1 and inside a thick part of a transmission case cover 0M3, through oil passages 02h and 03h passing through centers of the counter shaft 02 and the drive shaft 03, and is finally supplied to bearing sections 02f and 03f and mounting sections 02g, 03g of reduction gear trains 02e and 03e, to carry out lubrication of the bearing sections 02f and 03f, and the mounting sections 02g and 03g of the reduction gear trains 02e and 03e. 
The passage for supply of lubricating oil to the main shaft (counter shaft) 02 and counter shaft (drive shaft) 03 of the lubrication system of the transmission gear mechanism is made usually up of passages and slots formed inside wall sections of the transmission case 0M1 and inside a thick part of a transmission holder (transmission case cover) 0M3, and supply of lubricating oil to the main shaft 02 and counter shaft 03 is achieved by means of a fixed branched passage section from an engine main gallery.
The lubricating oil passages formed so as to pass through the inside of a wall section of the transmission case 0M1 and inside thick section of the transmission holder 0M3 are often formed as opened passage openings in end wall sections of the transmission case 0M1 and transmission holder 0M3, and passages for supply of lubricating oil are formed by blocking off the passage openings in an open state using suitable means, and lubricating oil is often supplied to the main shaft 02 or the counter shaft 03 through this type of branched passage section.
Blocking of the passage openings that are left open in the end wall sections of the transmission case 0M1 and the transmission holder 0M3 is usually achieved using plug members, specifically, by inserting plug members into passage openings that are opened and filling them, the openings are blocked up in a completely sealed state.
However, with this type of blocking of the passage openings that are in an opened state by inserting and filling plug members is troublesome operation, and it is necessary to prepare separate components such as the plug members for the blocking, which increases costs.
The lubricating oil supply passages of the lubrication system for the transmission gear mechanism have such an arrangement and structure, and passage openings of the end wall section of the transmission case 0M1 and transmission holder 0M3 that are to be blocked become the rear side of the plug members, that is, when looking from the side, transmission gear mechanism and the end wall section passage opening that are to be blocked, of the oil supply passage, are positioned completely overlapping the clutch members, which means that after clutch assembly it is difficult to confirm a blocked state of the passage opening of the end wall section using plug members or the like, and assembly errors of the plug members.
There has therefore been a demand for a good scheme for blocking of passage openings of end wall sections of the transmission case 0M1 and transmission holder 0M3 of a lubricating oil supply passage of a transmission gear mechanism that does not require a troublesome operation and is simple, and which can be ensured at low cost.